[Xorp-users] Avoiding asynchronous routing with OSPF

[Dirk H. Schulz]

Hi folks,

I have the following setup:

               -------------                  -------------
               |            |                 |            | 
upstream routers
               |            |                 |            |
               -------------                  -------------
                      |                              |
                      |                              |
                      |                              |
               -------------                  -------------
               |      1     |-----------------|      2     | 
my routers (connected via eth1)
               |            |                 |            |
               -------------                  -------------
                  |      |                        |      |
                  |      |                        |      |
                  |      |     -----------        |      |
                   -----------|          |--------       | 
switch A (subnet A)
                         |    |          |               |
                         |     -----------               |
                         |                               |
                         |     -----------               |
                         -----|          |---------------| 
switch B (subnet B)
                              |          |
                               -----------

The routers are using OSPFv2, the backbone area is the one behind my 
routers where subnet A and subnet B reside.

Let's call the interfaces going to the switches ethA and ethB on each 
router.

The servers are using virtual ips on ethA and ethB as gateways, so that 
servers in subnet A use router1 as gateway and servers in subnet B use 
router2 as gateway.

The problem I have is that packets from servers in subnet B go out via 
router2 and answers partially come back via router1. I tried to avoid this 
by setting interface costs:
1. The hightest interface cost is on eth1 which connects my routers 
directly (so data traffic between MY routers should be avoided).
2. The interface ethA without virtual ip address A (that is ethA on 
router2) has a significantly higher cost than the other; with ethB it is 
similar.

I had hoped that now OSPF would route the packets for subnet A always via 
router1 and for subnet B always via router2, but that does not work. There 
is still traffice destined for subnet B coming in on router1.

Is there anything I overlook? Is routing priority based on other parameters 
than interface cost additionally?

Any hint or help is appreciated.

Dirk